Separating Hot & Cold Water

One of those terrific science investigations that you're absolutely certain will just Not Work Out.... and then get to be surprised and thrilled when it does!

  1. Ages: 3 - 8

  2. <30 minutes

  3. A little messy


Step-by-step tutorial

  • Step 1

    First we prepped the index cards (yup, I said 'cards' because I figured we'd need backups!) by cutting them about 2" (5.1 cm) larger than our wide-mouthed jars. Easy enough!

    Photo reference of how to complete step 1

  • Step 2

    Next we had a practice run... best to do it intentionally rather than by accident! That way the mama can make sure her camera is out of the way before water goes splashing everywhere. Mmhmm. Onto the practice... we filled up a jar with water, put it in our safety pan, then over-filled it so it bulged on top a bit.

    Photo reference of how to complete step 2

  • Step 3

    We took a cut piece of index card, and carefully laid it on top of our jar, then tapped it lightly a few times to seal it to the jar. We picked up the jar, and quickly turned it directly upside down. All the way!, if it's angled, it may spill....

    Photo reference of how to complete step 3

  • Step 4

    Phew. Perfect! Okay, all three of us had mastered this step (the children love this part!), and were now confident, so let's do it for real... we filled one jar with very hot water, and the other with cold. Put a couple of drops of food coloring (er... we used liquid watercolor paints, as that's what we had) in each jar... red for hot and blue for cold.

    Photo reference of how to complete step 4

  • Step 5

    We paid close attention to how the color behaved.... the blue slowly moved, taking its time, while the red mixed quickly! (This happens because the hot water molecules are very active and moving fast.)

    Photo reference of how to complete step 5

  • Step 6

    We then filled up the cold the rest of the way, so that it bulged over the top of the jar. Laid on our index card, taped it down....

    Photo reference of how to complete step 6

  • Step 7

    ...and Maddie flipped it over quickly. Excellent! Worked again. We've so got this. Then comes a bit of tricky stuff.... we set the cold water jar on top of the hot, and placed them so the mouths of the jars lined up evenly. Holding on to the jars, we pulled out the index card. What happens?

    Photo reference of how to complete step 7

  • Step 8

    While this part may not surprise some of us, others might be thrilled with the magic of purple.... But it was very fast... even as we were still moving the card, we had just one color of water, now...

    Photo reference of how to complete step 8

  • Step 9

    Now let's do it all again, this time with blue on the bottom, and red (hot) on top. Remember to over-fill it, then tap on the index card. Turn it upside down, carefully pull out the index card from between them (keeping the jars together)...

    Photo reference of how to complete step 9

  • Done!

    And what happens this time? Woah! The hot and cold stayed separate. We watched it for at least five minutes, and it was still separate! We know the hot molecules are active, but here we learned too that the hot water has less density because of the activity, therefore the hot water is lighter than the cold... so hot stays on top. Before it had to rise to the top, through the cold--making the water purple. Science is cool. Wanna play?

    Photo reference of how to complete step 10

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